Placement of screws into the human spine is a common surgical procedure to allow for a multitude of spinal surgeries to be performed. Screws are typically placed into the pedicles of individual vertebra in the lumbar and sacral spine. Given their biomechanical advantages over other modes of fixation, surgeons are expanding the areas of the spine in which pedicle screws are placed. However, adjacent to the spine are numerous vital structures and organs, in particular the cervical and thoracic spine regions, which have very low tolerance for surgically created injuries that may ultimately lead to significant morbidity and/or mortality. For this reason the majority of research focus on placement of pedicle screws is centered on improving accuracy to maintain a screw within a bony (intraosseous) environment.
Image guided systems are evolving which are increasingly user friendly to assist a surgeon in accurately placing a screw. The critical parameters for placing a pedicle screw into the human spine are diameter, length, trajectory and then actual placement of the screw. To date many of the image guidance systems allow for manual determination of these parameters to improve a surgeon's manual performance in screw placement. Up to the present time, no system is available which will automatically determine ideal pedicle screw diameter, length and trajectory for accurate placement of pedicle screws. The present invention provides this capability akin to a pilot who flies an airplane with computer controlled aviation capabilities, and allows for placement of pedicle screws using either an open or percutaneous technique.
Patent Application Publication No. US 2004/0240715 A1, published on Dec. 2, 2004, relates to methods and computer systems for determining the placement of pedicle screws in spinal surgery. It discloses a method wherein the minimum pedicle diameter is first established for determining the optimum screw trajectory and then the maximum screw diameter and length using the optimum trajectory for each pedicle. Two dimensional transverse slice data is stacked to form three dimensional data points to determine optimum trajectory by linear least squares solution to fit the data, requiring the solution to go through the overall minimum transverse pedicle widths. A disadvantage of this method is that it allows for eccentric trajectory determination, particularly for distorted pedicle anatomy, with consequent smaller maximum diameter and length screw determinations resulting in biomechanically inferior constructions. In contrast, the new and improved method of the present invention always places the trajectory concentrically through the pedicle by the determination of optimum trajectory by using the center point of the smallest cross sectional area (isthmus) and projecting with a computer a line normal to this circumscribed area in opposite directions, as described more particularly hereinafter. The new and improved methods of the present invention allow for maximum screw diameter and length determinations for intraosseous placement.
In Patent Application Publication No. 2005/0192575-AL, dated Sep. 1, 2005, relating to methodology for the determination of ideal pedicle screw diameter, length and trajectory there is a description of the transitional interface where the pedicle is joined to the vertebral body. This transitional interface describes the pedicle base circumference (B) which is identified radiographically on anteroposterior radiographic imaging as a round like cortical density seen on the cephalad lateral aspect of the vertebral body. An essential feature of this pedicle base circumference is that it is different from the pedicle isthmus (X, the narrowest region within a pedicle), but can on occasion be the same. The pedicle isthmus is the rate limiting step to maximizing the largest diameter pedicle screw without causing a breach of the cortical wall. To maximize the diameter of the pedicle screw within any given pedicle the pedicle isthmus must be determined. Subsequently, the center of the pedicle isthmus allows determination of the ideal trajectory to allow for concentric pedicle screw placement along the ideal trajectory.
The present application is directed to new and improved methods for determining the pedicle base circumference, pedicle isthmus and center of the pedicle isthmus.